1. Field of the Invention
The present invention generally relates to methods of manufacturing semiconductor devices, and particularly relates to a method of manufacturing semiconductor devices which applies back-grinding to the back surfaces at the back-grinding stage.
2. Description of the Related Art
In recent years, portable terminals such as portable phones have been experiencing a rapid change of shapes towards increasingly compact and flat terminals. To this end, semiconductor devices used in such electronic equipment are expected to be flatter than ever.
In order to provide a flatter shape for semiconductor devices, the back surface of wafers are ground after circuitry is formed on the wafers. When the semiconductor devices are given a flatter shape through such a process, the structural strength will deteriorate.
Accordingly, there is a need to prevent chipping or cracking of semiconductor devices during the manufacturing process when a flatten shape is given to the devices.
FIGS. 1A through 1E are drawings showing an example of process steps concerning a related-art method of manufacturing a semiconductor device. FIGS. 1A and 1B show back-grinding steps that are performed after a wafer process in which circuits are formed on a wafer 1. FIGS. 1C and 1D show dicing steps, and FIG. 1E illustrates a mounting step.
As shown in FIG. 1A, the wafer 1 for which the wafer process is completed has a surface protection tape 2 adhered thereto. The surface of the wafer 1 on which the circuits are formed faces the surface protection tape 2. With this provision, the circuits formed on the wafer 1 are protected by the surface protection tape 2.
The wafer 1 having the surface protection tape 2 stuck thereon is mounted on a wafer chuck table 3 of a back-grinding apparatus. The back-grinding apparatus has a grinding head 4 that revolves. As shown in FIG. 1B, the back surface of the wafer 1 is rubbed against the grinding head 4, which achieves back-grinding of the back surface of the wafer 1 (i.e., back-grinding step).
When the wafer 1 is ground so as to have a predetermined thickness through the back-grinding step, as shown in FIG. 1C, the surface protection tape 2 is taken off from the wafer 1, and the back surface of the wafer 1 is stuck to a dicing tape 6. The dicing tape 6 is provided inside a frame 5, and has a surface thereof to which an adhesive (e.g., an adhesive that cures upon exposure to ultraviolet light) is applied so as to have the wafer 1 adhere thereto.
The wafer 1 stuck on the dicing tape 6 is carried to a dicing apparatus, where dicing is performed (i.e., a dicing step). The dicing is performed by cutting the wafer 1 through dicing lines provided on the wafer 1 in advance by use of a dicing saw 7. As a result, the wafer 1 is divided into pieces of semiconductor devices 10. Since the semiconductor devices 10 are stuck on the dicing tape 6 after becoming individual pieces, they are not scattered and lost by falling off from the dicing tape 6.
After the dicing step, the semiconductor devices 10 in the form of individual pieces are carried to a mounting apparatus together with the frame 5. In the mounting apparatus, ultraviolet light is shone first so as to reduce the strength of the adhesive that sticks the semiconductor devices 10. Thereafter, the semiconductor devices 10 are pushed up by pushing pins 11, so that the semiconductor devices 10 are separated from the dicing tape 6.
The semiconductor devices 10 taken off from the dicing tape 6 are sucked and captured by a collet 8, then carried to a board 9. The semiconductor devices 10 are mounted on the board 9, thereby completing the implementation process.
As described above, flatting of the wafer 1 is generally performed by mechanically grinding the back surface of the devices (FIG. 1B). At this back-grinding step, the grinding head 4 is rubbed against the back surface of the wafer 1. This results in minute scars being left on the finished back surface of the wafer 1, which are called back-grinding marks.
FIG. 2 is an illustrative drawing showing back-grinding marks. As shown in FIG. 2, back-grinding marks 12 are formed in the spiral form on the wafer 1. The back-grinding marks 12 remain even after the wafer 1 are divided into pieces of the semiconductor devices 10.
Consideration is now given to the shape of the back-grinding marks 12 formed on the pieces of the semiconductor devices 10. Dicing lines on the wafer 1 shown in FIG. 2 are supposed to extend in both the X direction and the Y direction.
As indicated by an arrow A in FIG. 2, a semiconductor device 10Y cut out of the wafer 1 has the back-grinding marks 12 generally extending in the Y direction. A semiconductor device 10X cut out of the wafer 1 as indicated by an arrow B in FIG. 2 has the back-grinding marks 12 generally extending in the X direction. In this manner, when the back-grinding marks 12 are generated on the wafer 1 in the spiral form, there will be always the semiconductor devices 10X and 10Y that have back-grinding marks 12 generally extending in the direction of dicing lines (i.e., the X direction and the Y direction).
FIG. 3 is an illustrative drawing showing the way the semiconductor devices are pushed up by pushing pins.
As shown in FIG. 3, the semiconductor devices 10X and 10Y having the back-grinding marks 12 thereon are taken off from the dicing tape 6 by use of the pushing pins 11. When this is done, a force F1 is applied to the semiconductor devices 10X and 10Y. This force F1 serves to bend the semiconductor devices 10X and 10Y as shown in FIG. 4 (i.e., forces F2 are applied as shown in FIG. 4).
The semiconductor devices 10X and 10Y has the back-grinding marks 12 that generally extend in the direction of dicing lines, i.e., in the direction parallel to the edges of the semiconductor devices 10. Compared with other pieces of the semiconductor devices 10, the semiconductor devices 10X and 10Y are thus weaker in terms of their structural strength, thereby possibly suffering chipping or cracking that originates from the back-grinding marks 12.
There is also a risk that a crystal defect such as cracked layers and micro cracks exists under the back-grinding marks 12. The back-grinding marks 12, the cracked layers, and the micro cracks may cause a chip crack or break.
The back-grinding marks 12 may also results in chipping being created on the wafer edges at the time of dicing the wafer. Such chipping may create further chipping or cracking.
When the wafer 1 having a thickness of 100 micrometers after back-grinding is divided into 8-mm-by-8-mm pieces of semiconductor devices 10, and the strength of pieces is measured through three-point bending test, for example, the test indicates an average of 2.8 N with a maximum of 3.4 N and a minimum of 2 N. When the semiconductor devices 10 are picked up by the pushing pins 11 (13 pins) from the dicing tape 6, the tolerable strength (which is required of the semiconductor devices 10 so as not to be broken) is known to be 1.8 N based on the computation and experiment.
This tolerable strength is very close to the minimum (2 N) of the strength of pieces obtained in the test, with only a little margin to spare. If the dicing tape 6 has a portion where adhesion is strong because of a variation in the effect of an adhesive, it is likely that the semiconductor devices 10 will suffer a crack.
It is thus desirable to take a countermeasure with respect to some of the semiconductor devices 10 that are weak in their structural strength, thereby increasing the minimum strength of the semiconductor devices 10. An example of such a countermeasure includes chemical etching by use of a chemical substance. Such a method requires a rather expensive facility and use of a large amount of chemical substance, thereby results in a significant const increase.
Accordingly, there is a need for a method of manufacturing semiconductor devices that can improve the structural strength of flattened semiconductor devices without increasing manufacturing costs.